In recent years, in accordance with the requirements of higher frequency and higher speed in the electronics field, the achievement of particularly low dielectric constant has been strongly demanded as the material characteristics of a resin substrate. Thus, porous resin materials have attracted much attention as resin substrate materials because of their low dielectric constant property, which is superior as compared with ordinary nonporous resin substrate materials.
In the past, proposals have been made with respect to the circuit connection materials and anisotropic conductive materials in which resin substrates are used. For example, an anisotropic conductive sheet is known in which electrical conductivity is afforded only to specific parts in a thickness direction by filling a conductive material in a number of through holes formed in the sheet made of a high polymer material. More specifically, a proposed anisotropic conductive sheet is formed in the following manner: a plurality of through holes are provided in an insulative stiff frame board made of a composite resin material which is reinforced with a resin material or glass fibers; and electrically conductive path elements are formed by filling the through holes with an insulating elastic macromolecular substance in which electro-conductive particles are dispersed. (Patent document 1)
Also proposed is an electrical connection member in which a metal is filled in a number of through holes formed in an electrically insulative macromolecular film such that electrical conductivity is afforded only in a thickness direction of the film (Patent document 2). Further, proposed is an elastic connector in which conductive members are arranged in a plurality of small holes formed in the thickness direction in an elastic sheet processed with foaming treatment (Patent document 3).
Generally, methods for providing perforations in a substrate are, for examples, machining methods such as punching by a punch and die, perforation by a mold, and drilling by a drill. Also known is a laser ablation method in which a laser is irradiated for perforation.
However, it is very difficult to apply the above-mentioned perforation methods to a porous resin material. In order to use the porous resin material as a substrate material, it is necessary to form a perforation which is greater than a pore diameter of the porous resin material. However, when a porous resin material formed in the shape of a substrate (hereinafter referred to as the “porous resin substrate”) is perforated by machining or laser processing, the porous structure of the wall face of a perforation tends to be destroyed to be nonporous. If the porous structure of the wall face of a perforation in the porous resin substrate is destroyed, the characteristics thereof as the porous resin material is impaired. The porous resin substrate has resilience in the thickness direction; however, when the porous structure of the wall face of a perforation is destroyed, the perforated part will be crushed and accordingly the resilience will be lost once a compressive load is applied in the thickness direction.
Moreover, even if a porous resin substrate is perforated, it will be very difficult to afford electrical conductivity by adhering metal selectively only to the wall face of the perforation in the secondary processing. Thus, it is difficult to accomplish a precise perforation of the porous resin substrate, and it is also difficult to perform the secondary processing after perforation.
A method proposed in the past for forming a via in a substrate includes the following process: at least one via is formed in the substrate by laser perforation, and subsequently, the cut-away substances generated as a result of the laser perforation are removed by a sodium etching treatment (Patent document 4). In Patent document 4, a porous organic substrate material which includes a fluoropolymer matrix is shown as the substrate. Patent document 4 teaches that the roughly cut-away substances which have re-adhered to the wall of the via during the laser perforation should be removed prior to the subsequent process of plating for metallic deposition. Patent document 4 discloses, as the porous organic substrate material which constitutes a substrate, a porous matrix system containing expanded polytetrafluoroethylene in which a mixture comprising a thermosetting or thermoplastic resin, an adhesive resin, and a filler is imbibed or impregnated.
[Patent document 1] Japanese Patent Application Publication No. H9-320667
[Patent document 2] Japanese Patent Application Publication No. H2-49385
[Patent document 3] Japanese Patent Application Publication No. 2003-22849
[Patent document 4] WO 98/20539
When a perforation is made in a porous resin material by laser processing, the porous structure of the wall face of the perforation tends to be destroyed, resulting in a nonporous structure. Likewise, when a perforation is made in a porous resin material by machining such as drilling or punching, the wall face of the perforation tends to become nonporous. If cut-away substances generated by laser processing for perforation remain, they can be removed by subjecting the wall face of the perforation to sodium etching.
However, if a fluoropolymer such as polytetrafluoroethylene (PTFE) is subjected to sodium etching treatment, the characteristics of PTFE will be compromised because a degenerative layer is thereby formed. Since the porous resin material formed from a resin material containing a fluoropolymer such as expanded porous PTFE has a micro-porous structure, it tends to suffer from the adverse effect of the degeneration due to the sodium etching treatment.
Particularly, an extremely difficult problem to be solved was how to make porous the part of the perforation wall face which has been made nonporous, in addition to maintaining the entire porous structure, when a perforation is formed by machining or laser processing in a porous resin material made of a resin containing a fluoropolymer. If the porous structure of the wall face of a perforation is left as destroyed, the characteristics thereof such as resilience will be hampered, and also it will not be possible to form a part having a sufficient electrical conductivity even if plating treatment is performed on the wall face of the perforation.